The higher accuracy of flow measurements with the aid of ultrasonic flow meters have imposed more stringent requirements on the resolution and data reliability of the frequency mixers used as output units in flow meters.
In a known method for mixing frequencies, a determination is made of the time intervals between pulses belonging to different pulse trains, which periodically vary in a linear manner from zero to a maximum value. These intervals are representative of the frequency difference period, hence of the frequency difference of two pulse trains.
This method is carried out by a prior art device comprising a flip-flop and a sawtooth voltage generator which incorporates a capacitor and a discharger (cf. N. I. Brazhnikov, Ultrasonic Techniques, Energiya Publishers, Moscow-Leningrad, 1965, pp. 167-169/in Russian/).
When the inputs of the flip-flop receive pulses of two trains, derived at its output are pulses whose width equals the time intervals between pulses from both trains. These pulses charge the capacitor to a threshold value, which is then discharged with the aid of the discharger. The frequency of the sawtooth voltages is representative of the frequency difference of two train pulses.
In the prior art device, however, the frequency difference determination may be erroneous due, in the case of a small frequency difference, to the phase instability of the pulse trains relative to each other. In addition, to ensure normal operation of the device, the trains of pulses applied to its inputs must be separated in time, which results in erroneous readings of real flow meters.
Another method for mixing frequencies provides discrimination of the frequency difference of pulse trains in the form of meandering shapes. Discriminated in accordance with this method is a frequency difference occurring as a result of a gradual change in the phase shift, the discrimination consisting in registering the presence of a pulse of one train at the moment of arrival of a pulse of the second train.
The prior art device for realizing this method comprises a clocked dynamic flip-flop, with the compared pulse trains being applied, respectively, to the data and clock inputs of the flip-flop, while frequency difference signals are derived at its output (cf. V. S. Gutnikov, Integrated Electronics in Measuring Instruments, Energiya Publishers, Leningrad, 1974, pp. 115-117, FIG. 62a/in Russian/).
The area of application of this method is limited by the shape of the pulses in the trains since it is applicable only to pulses of a meander shape. In this case, too, the readings obtained on the prior art device may be erroneous.
In still another method for discriminating the frequency difference of two pulse trains, the numbers of pulses in each train are compared. The instant at which the difference in the pulse number reaches an integral value is registered. Starting from that instant, a corresponding integral number of pulses of the greater frequency are allowed to pass, thereby providing a measure of the frequency difference.
This method is carried out in a device comprising means for comparing two pulse trains. The comparison means is built around a two-bit reversible counter with AND gates (cf. V. S. Gutnikov, Integrated Electronics in Measuring Instruments, Energiya Publishers, Leningrad, 1974, pp. 115-117, FIG. 62b/in Russian/).
The counter compares the numbers of pulses in each train, having frequencies f.sub.1 and f.sub.2. Once the difference in the number of pulses reaches an integral value, one of the AND gates disables further comparison. Therewith, the corresponding integral number of subsequent pulses of the greater frequency pass to the output of the device. Should the pulses of frequencies f.sub.1 and f.sub.2 coincide, the reversible counter may not operate properly, and the indication of the device will be false.
Since, when comparing two pulse trains, there always comes an instant at which pulses coincide, and that is not tolerable in the prior art method for discriminating the frequency difference of two pulse trains, the area of application of this method is very limited.
The means for comparing two pulse trains, built around a reversible counter with logic AND gates, functions normally only when the pulses applied to the input of the device are separated in time. A coincidence of pulses makes it inoperable, which results in erroneous information and improper operation of the device as a whole.